Tetrafluoroethylene polymers are of two types. One is non-melt-processible polymers where the melt viscosity is too high to process the polymers by ordinary melt-extrusion processes. Instead, the polymers are ordinarily sintered or paste extruded depending on the type of polymer. The other class is melt-processible tetrafluoroethylene copolymers having melt viscosities in the melt extrudable range.
Melt-processible tetrafluoroethylene copolymer resins obtained directly from the polymerizer and/or coagulator after drying are referred to as fluff or powder. The fluff is often treated and/or melt extruded to stabilize it. There are applications such as rotocasting in which a free-flowing, i.e., easily flowable, powder (herein called "granules") is more preferable than melt-extruded pellets, especially where a high degree of purity of the resin is desired. It would be desirable to provide melt-processible copolymer granules that are both stable and easily handled in a minimum of processing steps to avoid contamination from metallic processing equipment. It is particularly desirable to provide granules that can be used both in conventional melt-fabrication processes and in rotocasting applications.
It is also desirable to obtain resins that are thermally stable. A number of stabilization approaches are known in the art, most of which require melting the resins. Thus resins stabilized by these methods are generally available only as pellets--not (without tedious and expensive regrinding steps) as the free-flowing granules that are the basis of this invention.
Another desirable feature of such resins is that the granules should be low in metal contamination. If the granules have been melted in conventional thermoplastic processing equipment, contamination occurs inevitably when the corrosive tetrafluoroethylene copolymer melts come in contact with the interior metal surfaces of thermoplastic processing equipment, even when corrosion-resistant alloys are used. Copolymers having low levels of metal contamination are particularly desirable for applications in the semiconductor industry.